Densification of irradiated metal

ABSTRACT

A method of producing dense bodies of irradiated metal in various geometrical shapes to achieve near theoretical density specimens of such metal is presented. These specimens are excellent radiation sources of high specific activity for medical therapeutic and industrial radiographic applications. The process starts with a target metal which is encapsulated in a metallic jacket and irradiated with a flux sufficient to convert a substantial portion of the target metal to a radioisotope. The irradiated metal is removed from the capsule and a given amount of the irradiated metal sufficient to give a known energy output is segregated and either (a) arc melted or (b) inductively cast to achieve a desired configuration as a radiation source. The process gives irradiated metal with a very small focal point and a very high density.

United States Patent 1 France et al.

[ June 26, 1973 DENSIFICATION OF IRRADIATED METAL [75] Inventors: Delmar W. France, Fremont; Arlo D.

Ketcham, Livermore, both of Calif.

[73] Assignees General Electric Company, San Jose,

Calif.

[22] Filed: Nov. 18, 1970 [2]] Appl. No.: 90,517

Primary Examiner-Archie R. Borchelt Attorney-Ivor J. James, Jr., Samuel E. Turner, Sam E.

Laub, Frank L. Neuhauser, Oscar B. Waddell and .loseph B. Forman [57] ABSTRACT A method of producing dense bodies of irradiated metal in various geometrical shapes to achieve near theoretical density specimens of such metal is presented. These specimens are excellent radiation sources of high specific activity for medical therapeutic and industrial radiographic applications. The process starts with a target metal which is encapsulated in a metallic jacket and irradiated with a flux sufficient to convert a substantial portion of the target metal to a radioisotope. The irradiated metal is removed from the capsule and a given amount of the irradiated metal sufficient to give a known energy output is segregated and either (a) are melted or (b) inductively cast to achieve a desired configuration as a radiation source. The process gives irradiated metal with a very small focal point and a very high density.

11 Claims, 2 Drawing Figures Eacapsulate Target Metal in Metallic Jacket lrradiate Target Metal L Remove Irradiated Metal from Metallic Jacket Measure Desired Weight of Irradiated Metal 1 l iolrdity the lrradlated Metal PAIENIEDJUIZS ms 3.742.366

sum 1 at 2 Encapsulate Target Metal in Metallic Jacket Irradiate Target Metal Remove Irradiated Metal from Metallic Jacket Measure Desired Weight of Irradiated Metal Liquify the Irradiated Metal Salidify the Irradiated Metal Fig.

INVENTORS DELMAR W. FRANCE ARLO D. KETCHAM ATTORNEY PATENTEDJUIZG I975 sumaurz WEIGHT OF COBALT PELLETS IN GRAMS Development of production-size gamma irradiation facilities has created a fresh approach to industrial processing that meets the demand of modern manufacturers. This equipment provides a tool of controlled irradiation intensity with the potential for creating new products and new processing operations.

Irradiation produces changes in materials that are often impossible to duplicate with other manufacturing methods. Through a process called ionization, energy emitted from radioactive isotopes causes certain effects that are desirable in a variety of manufacturing and processingoperations.

For example, ionization kills bacteria and produces chemical changes in many organic substances. Thus, irradiation processing can be used for a variety of industrial and other purposes, such as sterilizing medical supplies and foodstuffs, increasing the storage life of fresh foods, improving the physical properties of commercial plastics and efficiently producing bulk chemicals.

One of the most attractive forms of ionizing energy used in large scale production is gamma irradiation. It utilizes gamma rays emitted from a variety of radioactive isotopes. Of these, cobalt-60 seems particularly well suited for industrial and other uses.

The cobalt-60 radioisotope is widely used for medical therapy, especially cancer therapy. In cancer therapy, the cobalt-60 source must have a configuration which furnishes a high intensity output and a small localized focal point. This requires a cobalt-60 source having a small volume with high intensity output per given volume.

Current methods of fabricating sealed sources from irradiated metal include making wafers (flat disks of 1 mm. in thickness and 1 to 2 cm. in diameter) and stacking the wafers in a source body (usually a stainless steel cylindrical can). Another method involves placing a given weight of loosely packed pellets of irradiated metal in a source body (again usually a. stainless steel cylindrical can). The use of wafers limits the configuration of the sealed source which limits the flexibility in final intensity output of the sealed source. Wafers of target metals have poor activation capabilities, i.e., poor ability to absorb neutrons as a target material during exposure to neutron fluxes. To overcome the drawbacks with wafers, the cylindrical can is filled with irradiated pellets (right cylinders of 1 mm. in diameter and 1 mm. in height). This gives more flexibility to the configuration of the source and gives good activation capabilities. But the use of pellets introduces the problem of low density of the irradiated metal per given volume due to the uniform shape of the pellets.

Use of fabrication techniques involving melting has been avoided by the prior art due to problems with contamination of the surrounding environment with the irradiated metal (called contamination spread) and contamination of the irradiated metal itself such as with oxides.

Accordingly it has remained desirable to have a process for fabricating dense geometrical shapes of irradiated metal to achieve near theoretical density specimens. This process should include the desirable features of the prior art process using pellets, namely good activation of the target material and flexible configuration of the irradiated metal.

SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide irradiated metallic specimens of a high density, preferably a density of at least 95 percent of theoretical density.

Another object of this invention is to achieve irradiated metallic specimens having very small focal points to enable concentration of the radiation intensity.

A still further object of this invention is to provide a process for producing a dense cobaltradioisotope in various configurations while minimizing contamination spread and contamination of the cobalt-6O.

The foregoing and related objects will be apparent to a person skilled in the art from a reading of the following specification and the appended claims.

It has been discovered that irradiated metallic specimens having very high density and possessing desirable radiation focal points can be achieved using the process of the present invention. The process starts with particles of the metal in an unirradiated condition (target metal) and encapsulates the particles of target metal in a thin metallic annulus promoting the absorption of neutrons throughout the target metal. During the remainder of the process the target metal is enclosed in shielded zones to confine radioactive energy release. Next the encapsulated target metal is subjected to irradiation sufficient to enable some of the atoms of the metal to capture neutrons and the irradiated metal is removed from the aluminum encapsulation. A given weight of the irradiated metal sufficient to provide a known irradiation output is measured and the irradiated metal is subjected to sufficient thermal energy to be liquified and then solidified in a desired shape. The irradiated metal thus treated is ready to be positioned in a source holder.

The important steps of the process are the liquification and solidification steps which achieve the densification and impart the desired shape to the irradiated metal. It has been found that there are two preferred practices of these steps as follows: (l) are melting the irradiated metal with solidification in a crucible of desired shape or (2) induction casting of the irradiated metal with solidification in a crucible of desired shape. These two preferred practices produce irradiated metal specimens of very high densities.

DETAILED DESCRIPTION OF THE INVENTION In greater detail, the present invention provides a process for the densification of any metallic radioisotopes with a melting point up to about 2,000C to provide approximately theoretical density specimens of such irradiated metal in various configurations. This process enables an improved radioisotope product for V use in sealed sources having configurations giving very small radiation focal points of very high intensity.

In general the process will be described by using underlined headings corresponding to the process steps given in the flow chart of FIG. 1. In general the process begins with the unirradiated commercially available metal (target metal) corresponding to the desired metallic radioisotope (e.g., cobalt-59 when cobalt-60 is the desired radioisotope and iron-58 when iron-59 is the desired radioisotope). The target metal may have one or more coatings on the metal especially where it is desirable to prevent oxidation of the metal or possible loss of the metal such as through vaporization. It is preferable to have the metal in the form of dense pellets such as platelets or small cylinders and metals unavailable in this form such as powders are converted to such compacted pellets. The target metal is preferably in the form of pellets having a geometrical configuration of a right cylinder of 1 mm. in diameter by 1 mm. in height.

ENCAPSULATE TARGET METAL IN METALLIC JACKET The metal pellets are first encapsulated in a metal jacket such as between two concentric aluminum cylinders to form a capsule with the thickness of the metal pellet annulus being about 0.050 to about 0.100 inches, The aluminum cylinders are joined by welding. The aluminum cylinders enable the metal pellets to absorb neutrons throughout the metal whereas with geometrical configuration of thicker cross sections, the metal pellets would not be uniformly activated (pellets in the center of thick cross sections would not absorb neutrons as readily). In general it is preferred that the aluminum cylinders be about 0.03 to about 0.04 inches in thickness.

From this point careful handling of the capsule with the included metal is necessary as the metal becomes radioactive in later processing. Accordingly, the capsule is preferably kept in shielded zones and manipulated by conventional remote control instruments and manipulators.

IRRADIATE TARGET METAL REMOVE METALLIC RADIOISOTOPE FROM METALLIC JACKET The metallic radioisotope is removed from the capsule by various means such as by cutting through the aluminum jacket and prying open the jacket so that the radioisotope pellets are released (again by use of remote control instruments in a shielded zone due to the radioactive nature of the irradiated metal).

MEASURE DESIRED WEIGHT OF RADIOISOTOPE The ultimate use of the irradiated metal will dictate the quantity of metal needed to give a known intensity output. For example for radiography applications a sealed source is used which has up to about 4 grams of irradiated metal such as cobalt-60 in a given configuration sealed in a stainless steel holder. Accordingly the desired weight of irradiated metal is measured for liquification and solidification in the desired shape for the sealed source.

LIQUIFY AND SOLIDIFY THE RADIOISOTOPE The desired weight of the irradiated metal pellets is solidified to a given shape by a liquificationsolidification sequence which achieves densification. The radioisotope pellets are subjected to sufficient thermal energy to be liquified and are then solidified in a desired shape. The metal is heated in one practice to a sufficient temperature by arc melting in a metallic or ceramic crucible using a heliarc torch under a controlled atmosphere. In another practice the irradiated metal is induction cast at a sufficient temperature in a metallic or ceramic crucible with the heating being done by inductive coils surrounding the crucible under a controlled atmosphere.

The liquification and solidification are conducted in hot cells which are highly shielded, controlled atmosphere enclosures with lead glass Windows to enable viewing the process. The hot cells are equipped with externally manipulated mechanical instruments for conducting the process. The incoming atmosphere is controlled for moisture content and the atmosphere withdrawn from the hot cell is run through a high efficiency filter to remove all radioactive particles. The liquification and solidification steps are conducted within controlled temperature ranges generally no higher than about C above the melting point of the metallic radioisotope to avoid vaporization of the radioisotope.

An especially preferred process of the present invention is the conversion of cobalt-59 to cobalt-60. The process begins using a metallic coated cobalt-59 such as a nickel coated cobalt59 which is in the form of pellets preferably in the shape of right cylinders having the dimensions of 1 mm. in height and 1 mm. in diameter. The pellets of cobalt-59 are encapsulated between two concentric aluminum cylinders with the thickness of the cobalt annulus being about 0.050 to about 0.100 inches. The encapsulated cobalt-59 is irradiated with a flux of at least about 4 X 10 neutrons per square centimeter per second for a time period of about 1 year to about 3 years. After irradiation the cobalt is removed from within the aluminum encapsulation. A given weight of irradiated metal sufficient to give a known irradiation intensity output is segregated, liquified and solidified to the configuration desired. The liquification and solidification are accomplished by either (1) arc melting the cobalt-60 in a copper crucible (e.g., a crucible having a spherical shape) using a heliarc torch at a temperature of about l,500 to about 1600 C under a controlled atmosphere (e.g., argon, nitrogen, helium) or (2) induction casting the cobalt-60 in a ceramic (Al- O crucible with heating being done by high frequency inductive coils surrounding the crucible at a temperature of about l,500 to about 1,600 C under a controlled atmosphere (e.g., argon or vacuum). The cobalt radioisotope is then allowed to solidify in the crucible.

The liquification and solidification are conducted in a hot cell which is a highly shielded, controlled atmosphere enclosure with a lead glass window to enable viewing the process. The hot cell is equipped with externally manipulated mechanical instruments for conducting the process. The incoming atmosphere is controlled for moisture content and the atmosphere withdrawn from the hot cell is run through a high efficiency filter to remove all radioactive particles. The liquification and solidification steps are conducted within the foregoing temperature range to avoid vaporization of the cobalt-60. The solidified cobalt-60 shape is ready to be placed in a source holder such as a stainless steel source holder.

FIG. 2 presents the relationship between the size of the metallic radioisotope body and the weight of the radioisotope in the body for a spherical configuration when the irradiated metal is cobalt. The irradiated metal containing bodies for which this relationship is presented are produced by are melting.

in the following portion of the specification, an example of the process of this invention is given for the purpose of more fully illustrating the invention and is not to serve in any manner as a limitation on the teaching of the invention.

EXAMPLE Fifty. grams of nickel coated cobalt-59 pellets in the form of right cylinders of 1 mm. in diameter and 1 mm. in height were encapsulated between two hollow aluminum cylinders to form a capsule with the pellet annulus being 0.050 inches in thickness. The outer aluminum cylinder has an outside diameter of 1.125 inches and an inside diameter of 1.095 inches and the inner aluminum cylinder has an outside diameter of 0.995 inches and an inside diameter of 0.965 inches. The height of the capsule is 4 inches. The capsule was then placed in an irradiation chamber and irradiated with a flux of 4.5 X neutrons per square centimeter of surface area of the metal per second for one year. Using remotely operated tools on the capsule in the chamber, the cobalt was removed from the aluminum encapsulation. Four samples were then taken from the 50 grams of irradiated pellets. The weight of irradiated cobalt for each sample is given in the table below. Samples A and B were are melted in a copper crucible at a temperature in the range of l,500 to l,530C under an argon atmosphere using a heliarc torch. Samples C and D were induction cast in an alumina crucible at a temperature in the range of l,500 to 1,530C under vacuum. Heating of the alumina crucible is done by high frequency inductive coils surrounding the crucible. The following table gives the resulting densities of the four samples of cobalt-60 as a percent of theoretical density.

TABLE lnitial Initial density weight of sample Final density of as pellets percent as percent of in of each theoretifinal weight theoretical sample cal Sample (grams) density of samples density A 342] 55% .3421 96% B .3481 55% .3445 96% C 3470 55% .34l9 96% D 3448 55% .3432 96% It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

What is claimed is:

l. A method of producing dense specimens of metal lic radioactive bodies from a corresponding target metal comprising the steps of a. encapsulating the target metal in a metallic jacket to form a thin cross section of the target metal,

b. irradiating the encapsulated target metal with a neutron flux sufficient to convert a substantial portion of the target metal to a radioisotope,

c. removing the irradiated metal from the metallic jacket,

d. liquifying a measured weight of the irradiated metal sufficient to give a known radiation output, and

e. solidifying the irradiated metal in the desired configuration.

2. A method according to claim 1 in which the liquification step is performed by are melting the metal in a crucible under a controlled atmosphere.

3. A method according to claim 1 in which the liquifcation step is performed by induction casting the metal in a crucible under a controlled atmosphere.

4. A method according to claim 1 in which the percursor metal is cobalt-59 and the radioisotope is cobalt 60.

5. A method according to claim 1 in which the percursor metal is iron-58 and the radioisotope is iron-5 9.

6. A method of producing dense specimens containing metallic radioisotopes of desired configuration from a given weight of pellets of the irradiated metal comprising the steps of a. liquifying the given weight of pellets of the irradiated metal, and

b. solidifying the irradiated metal in the desired configuration.

7. A method according to claim 6 in which the liquification step is performed by arc melting the irradiated metal in a crucible under a controlled atmosphere.

8. A method according to claim 6 in which the liquification step is performed by induction casting the irradiated metal in a crucible under a controlled atmosphere.

9. A method according to claim 1 in which the irradiation of the target metal is conducted at a flux of at least about 4 X 10 neutrons per square centimeter per second.

10. A method according to claim 1 in which the liquification step is conducted at a temperature in the range of about l,500 to about l,600C for an irradiated metal of cobalt.

11. A method according to claim 1 in which the metallic jacket is comprised of aluminum.

'l ST -ATES PATENT OFFICE l I IY EIOF 'C RRECH N Pqtent Noll 3'74'21366 Dated 26. June 1973 if fl D. W, France/A, D. Ketcham '2 is certified that error appearsv in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 52,, delete "metallic"? and. replace with irradiated metal containing- Column 2,v line .56, delete "radioisotope"; Column 3, ;liine- 16, the comma after inches should be a" period. Column 4, line '30, "cobalt5l9" should be ---cobalt-5'9"-l--; Column 5, line 4,, delete "metallic radioisotope"; Column 5 lines 4 &' 5, delete v "radioisotope," and replace with -irradiated metal; Column 6, line 8,

' before the comma add -V---promoting absorption of neutrons";

Column '6, line 17, the'se-cond occurrence of "the" should; be

; --a---; Column 6, lines & '26, delete "percursor" and replace with --target- Coltlrg nli} lines 28 & 29 delete percurslrir and replace with; --taret-L-; Column 6, line 33, replace the Signed and sealed this 8th day of October 1974-0 KSEAL) fittest:

IEIcCOY M," GIBSON JR. Cw MARSHALL DANN {Xttesting Officer 7 Commissioner of Patents :FQR i' (we?) 7 USCOMM-DC B0376-PB9 9 U.5. GOVERNMENT PRHTING OFFICE: 1959 0-365-33 MCCOY Mu GIBSON JR. {Xttesting Officer I {UNITED STATES PATElll oFflcE I f- "CERTIFICATE 10F CORRECHQ 26- June 1973 Petent No. 3,742,366 Dated D. Wo France/A, Do Ketcham It is certified that error appears. in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 52 delete. 5"metallic? and. replace with irradiated metal containing- Column 2, line 565, delete "radioisotope"; Column 3,, li:ne 16, the. comma after inches should be a period, Column 4; line 130,, cobalt59" should be: '--cobalt-5'91--; Column 5, line 4,, delete metallic radioisotope"; Column 5, lines 4 &' 5, delete "radioisotope." and replace with --irradia.ted meta:l--; Colunm -6, line 8,, I

' before the comma add ---promoting absorption of neutrons' -g Column 6; line .17, the second occurrence of "the" should be Column 6, ;lines'% & 26, delete w ercursofland replace with --ta;:get- -f; Colurgnfi} lines 28 s. 29 delete "pe'rcursor" and. replace with ---tar?et'-i-; Column 6, line 33, replace the Signed and sealed this 8th day of October 197% {sat-XL) httest:

Co MARSHALL DANN Commissioner of Patents USCOMM-DC 5O37fi-P59 11.5. GOVERNMENT PRINTING OFFICE: I969 0-356-334 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,742,366 Dated 26 June 1973 I...-e-:.:or(s) D. W. France/A. D. Ketcham *t is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 52, delete "metallic" and replace with irradiated metal containing- Column 2, line 56, delete "radioisotope"; Column 3, line 16, the comma after inches should be a period. Column 4, line 30, "cobalt59" should be --cobalt-59--; Column 5, line 4, delete "metallic radioisotope"; Column 5, lines 4 & 5, delete "radioisotope" and replace with --irradiated metal--; Column 6, line 8, before the comma add --promoting absorption of neutrons--; Column 6, line 17, the second occurrence of "the" should be --a-'-; Column 6, lines & 26, delete "percursor" and replace with -target-; ColumnG, lines 28 & 29, delete "percursor" and replace with --tar@et--; Column 6, line 33, replace "the" with -an--..

Signed and sealed this 8th day of October 1974 (SEAL) Attest:

McCOY M. GIBSON JR. (3. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-OC 503784 59 w U s COdiRNMENT PRINTiKG OFFICE- I989 03e6-u4 

2. A method according to claim 1 in which the liquification step is performed by arc melting the metal in a crucible under a controlled atmosphere.
 3. A method according to claim 1 in which the liquification step is performed by induction casting the metal in a crucible under a controlled atmosphere.
 4. A method according to claim 1 in which the percursor metal is cobalt-59 and the radioisotope is cobalt-60.
 5. A method according to claim 1 in which the percursor metal is iron-58 and the radioisotope is iron-59.
 6. A method of producing dense specimens containing metallic radioisotopes of desired configuration from a given weight of pellets of the irradiated metal comprising the steps of a. liquifying the given weight of pellets of the irradiated metal, and b. solidifying the irradiated metal in the desired configuration.
 7. A method according to claim 6 in which the liquification step is performed by arc melting the irradiated metal in a crucible under a controlled atmosphere.
 8. A method according to claim 6 in which the liquification step is performed by induction casting the irradiated metal in a crucible under a controlled atmosphere.
 9. A method according to claim 1 in which the irradiation of the target metal is conducted at a flux of at least about 4 X 1014 neutrons per square centimeter per second.
 10. A method according to claim 1 in which the liquification step is conducted at a temperature in the range of about 1,500* to about 1,600*C for an irradiated metal of cobalt.
 11. A method according to claim 1 in which the metallic jacket is comprised of aluminum. 